This invention relates to an image density control device for controlling the density of an image formed on copy paper in a copying machine or like image forming apparatus.
Conventionally, an image density control device for use in an image forming apparatus such as a copying machine has been designed to control the density of an image formed on copy paper (hereinafter referred to as image density throughout the Specification) by changing an amount of light emitted from a lamp to be projected onto a document while maintaining the surface potential of a photosensitive drum at a fixed level.
For example, in a copying machine provided with an automatic exposing device of the prescanning type, a predetermined amount of light is emitted from a lamp and projected onto a document, and a prescanning operation is executed while the light reflected by the document is being detected by a photodetector. The density of the document (document density) is calculated on the basis of the amount of light detected in the prescanning. Based on the calculated document density, an amount of light to be emitted from the lamp is determined in a main scanning operation wherein copying of an image is actually executed.
Further, when the image density is manually adjusted, for example, the light emission amount of the lamp is determined according to the document density set by manipulating a density setting key with the use of a conversion table in which the relationship between the document density and the light emission amount of the lamp is predefined.
FIG. 5 is a graph showing a conventional relationship between the document density and the light emission amount of the lamp, according to which the light emission amount is determined based on the document density. In FIG. 5, indicated at A, B, C are reference document densities corresponding to reference documents G1, G2, G3. For instance, when the detected or set document density is substantially equal to that of the reference document densities A, B, or C, light emission amount of the lamp is a, b, or c.
In a conventional image density control device, the lamp emits less of light as the document density becomes lower. Accordingly, when an image of a document whose density is lower than the density B is to be copied, there is a likelihood that the amount of light emitted from the lamp is too small relative to the actual density of the document, thereby causing the fog on the copy to have higher density than necessary. Hereinafter, the density of the fog is referred to as fog density. In other words, a white area in the document is formed into a grey area in the copy. In order to prevent the fog density from increasing to an unnecessarily high level, it may be considered that the relationship between the document density and the light emission amount is defined in such a fashion as to make the inclination of the curve in the graph of FIG. 5 smoother below the density B. Thereby, the light emission amount of the lamp can be adjusted to be larger than the conventional light emission amount in the case where the document density is relatively low. However, in this case, the following problem occurs. If the density of a document image is low, the resulting image density is low as a whole, thereby forming an unclear image. This is especially prominent if the document image consists of characters, or a diagram.
On the other hand, when an image of a document whose density is higher than the density C of the reference document G3 is to be copied, the light emission amount of the lamp is c, which corresponds to the reference document G3. Similar to the foregoing case, there is a likelihood that the fog density increases to a higher level than necessary due to deficiency in light emission amount. In order to avoid this problem, the use a lamp capable of emitting a larger amount of light may be considered. However, this results in increased costs and an increase in the temperature of a scan/exposure system due to the heat generated from the lamp, thereby necessitating implementation of some measures against the generated heat.